1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device widely used for OA or AV equipment, and a driving method thereof.
2. Description of the Background Art
A liquid crystal display device generally has a structure such that, a liquid crystal layer is sandwiched between an opposed electrode and a pixel electrode. In particular, in an active matrix liquid crystal display device, numbers of pixel electrodes are arranged in a matrix. To display an image in the active matrix liquid crystal display device, a picture signal is successively provided to the numbers of pixel electrodes arranged two-dimensionally in a matrix, on a column-by-column basis. Hereinafter, this process is referred to as “scanning”. At a timing when the scanning reaches a certain column of the pixel electrodes, a voltage as a picture signal is applied simultaneously to all of the pixel electrodes belonging to the column, and a potential difference held at that time in each pixel electrode for the opposed electrode must be kept to a sufficient degree until next scanning comes to the column of the pixel electrodes. As a capacitance (which is also referred to as “a liquid crystal capacity”, generally indicated with “Clc”) resulting from a liquid crystal between the pixel electrode and the opposed electrode is usually not sufficient, a technique is known to give the pixel electrode an auxiliary capacity (generally indicated with “Cs”) to help charge keeping of the pixel electrode. More specifically, to give the auxiliary capacity, it is contemplated that a portion of the pixel electrode is arranged so as to overlap with another wiring or the like with interposed dielectric.
Techniques disclosed in Japanese Patent Laying-Open No. 10-274783 and No. 7-311390 are examples of conventional techniques regarding the auxiliary capacity. In these techniques, one pixel region is divided into a plurality of pixel electrodes, and one of the pixel electrodes has a portion overlapping with capacity wiring to achieve an auxiliary capacity as a so-called Cs on Common, while another pixel electrode has a portion overlapping with a gate signal line for an adjacent pixel to achieve an auxiliary capacity as a so-called Cs on Gate. Each of the divided pixel electrodes has a TFT (Thin Film Transistor) element for driving. An object of the techniques in Japanese Patent Laying-Open No. 10-274783 and No. 7-311390 is to improve a visual angle characteristic of a liquid crystal display device by providing a plurality of regions having different auxiliary capacity components within one pixel.
Other relational conventional techniques include Japanese Patent Laying-Open No. 7-218930 and No. 9-15622 as examples of Cs on Common, and Japanese Patent Laying-Open No. 8-146464 and No. 2000-227611 as examples of Cs on Gate.
Generally, auxiliary capacity Cs is achieved with the aforementioned two systems, Cs on Gate and Cs on Common.
FIG. 17 shows a circuit diagram of Cs on Gate. Cs on Gate means that the auxiliary capacity is retained between a pixel electrode or an auxiliary electrode having the same potential as the pixel electrode and a gate signal line Gn−1 for an adjacent pixel region. A portion enclosed with dotted line in FIG. 17 indicates a position in the circuit construction occupied with the pixel electrode or the auxiliary electrode having the same potential as the pixel electrode.
FIG. 18 shows a circuit diagram of Cs on Common. Cs on Common means that the auxiliary capacity is retained between a pixel electrode or an auxiliary electrode having the same potential as the pixel electrode and auxiliary capacity wiring Cs. Auxiliary capacity wiring Cs is wiring arranged between gate signal lines to achieve Cs on Common structure. Thus, the gate signal lines and auxiliary capacity wirings are alternately arranged one by one.
Both of the two systems Cs on Gate and Cs on Common have respective merits and demerits, and are selected corresponding to specific needs. One of the merits of Cs on Gate is that, there is no need to arrange a new capacity line, as a gate signal line of an adjacent pixel of each pixel is used as wiring to achieve the auxiliary capacity. In addition, as an arrangement of an additional capacity line is not required, an aperture ratio is not decreased. One of the demerits, however, is that, as a load for auxiliary capacity Cs is applied to the gate signal line of the adjacent pixel, power supply efficiency during a gate-off time (a state of a voltage being VGL) becomes lower and a current consumption is increased. On the other hand, one of the merits of Cs on Common is a small current consumption. One of the demerits is a decreased aperture ratio, as the auxiliary capacity wiring must be additionally arranged only for achieving the auxiliary capacity.
As a liquid crystal has permittivity anisotropy, liquid crystal capacity Clc varies depending on a voltage applied to the liquid crystal. Thus, a direct current component originally held by a potential of the pixel electrode changes with the picture signal. As a result, an undesired direct current component is newly generated. When a direct current voltage is applied to the liquid crystal by the direct current component generated as such, a flicker (flickering or shaking of a screen) occurs. In addition, if the direct current component is generated, an after image is formed, which may be resulting from formation of an electric double layer on an interface between an alignment layer and the liquid crystal. Furthermore, a response property of a picture is known to be degraded because liquid crystal capacity Clc changes with the applied voltage.
Sufficiently large auxiliary capacity Cs can suppress the above-described problems caused by a variation in liquid crystal capacity Clc and can enhance performance of the liquid crystal display device. The auxiliary capacity, however, is not sufficiently large in any of the techniques in aforementioned six references.